Abstract

Many studies have reported inverse associations between vegetable and
fruit consumption and lung cancer risk. The aim of the present study
was to elucidate the role of several antioxidants and folate in this
relationship. In the Netherlands Cohort Study on Diet and Cancer,
58,279 men of ages 55–69 years at baseline in 1986 returned a
questionnaire including a 150-item food frequency questionnaire. After
6.3 years of follow-up, 939 male lung cancer cases were registered. A
new Dutch carotenoid database was used to estimate intake ofα
-carotene, β-carotene, lutein + zeaxanthin, β-cryptoxanthin, and
lycopene, completed with the antioxidant vitamins C and E and folate.
Using case-cohort analysis, rate ratios were calculated, adjusted for
age, smoking, educational level, and family history of lung cancer.
Protective effects on lung cancer incidence were found for lutein +
zeaxanthin, β-cryptoxanthin, folate, and vitamin C. Other carotenoids
(α-carotene, β-carotene, and lycopene) and vitamin E did not show
significant associations. After adjustment for vitamin C, only folate
remained inversely associated, and after adjustment for folate, onlyβ
-cryptoxanthin and vitamin C remained significantly associated.
Inverse associations were strongest among current smokers and weaker
for former smokers at baseline. Inverse associations with carotenes,
lutein + zeaxanthin, and β-cryptoxanthin seemed to be limited to
small cell and squamous cell carcinomas. Only folate and vitamin C
intake appeared to be inversely related to small cell and squamous cell
carcinomas and adenocarcinomas. Folate, vitamin C, andβ
-cryptoxanthin might be better protective agents against lung cancer
in smokers than α-carotene, β-carotene, lutein + zeaxanthin, and
lycopene.

Introduction

Vegetables and fruits are associated with a reduced risk of
cancers, especially lung cancer (1, 2, 3, 4)
. Recent results
from the Netherlands Cohort Study on Diet and Cancer on vegetable and
fruit consumption and the association with cancer incidence at several
sites have revealed reduced risks for lung cancer (5)
,
less pronounced effects on colorectal cancer (6)
and
stomach cancer (7)
, and no effect on prostate cancer
(8)
.

The possible protective compounds in vegetables and fruit include a
wide variety of phytochemicals (9)
. Among them are the
carotenoids, colorful compounds that are abundant as pigments in
plants. The main carotenoids are α-carotene, β-carotene, lutein,
zeaxanthin, β-cryptoxanthin, and lycopene. They are potent quenchers
of free radicals, which are by-products of metabolic processes
originating from environmental pollutants such as cigarette smoke. Some
carotenoids can be metabolically converted into retinol (α-carotene,β
-carotene, and β-cryptoxanthin). Case-control studies on the
relationship between carotenoids in food and the incidence of lung
cancer have shown inverse associations with β-carotene, α-carotene,
and lutein + zeaxanthin but not with lycopene or β-cryptoxanthin
(10, 11)
. In the past decade, eight prospective studies
have been published concerning the association of dietary carotenoids
and other antioxidants with lung cancer risk (12, 13, 14, 15, 16, 17, 18, 19)
. Six
of them presented RRs3
using a summed variable for carotenoids (12, 14, 15, 17, 18, 19)
; one of them also presented mean daily intakes for the
major carotenoids for cases and non-cases (12)
. Four
studies presented RRs separately for β-carotene
(13, 14, 15, 16)
, and one recent study presented RRs for all major
carotenoids (19)
. In the analyses, other antioxidants were
included such as vitamin E (12, 16, 17, 18, 19)
and vitamin C
(12, 13, 14, 15, 16, 17, 18, 19)
, and two studies included folate (18, 19)
. The studies were performed in men (12, 14, 16)
, women (15, 19)
, or both (13, 17, 18)
. Most studies presented RRs for the respective food
constituents by smoking status (12, 15, 17, 18)
and/or by
histological type of lung cancer (15, 18)
.

We have recently developed a database containing data on the most
important carotenoids based on chemical analyses of the main vegetables
and literature values (20)
. This gave us the opportunity
to perform analyses on lung cancer incidence and carotenoid intake in
the Netherlands Cohort Study on Diet and Cancer and to evaluate in more
detail possible mechanisms underlying the inverse associations found
between vegetable and fruit consumption and the incidence of lung
cancer (6)
. In addition to the carotenoids covered by the
database (α-carotene, β-carotene, lutein + zeaxanthin,β
-cryptoxanthin, and lycopene), folate and the antioxidant vitamins C
and E were included in the analyses. With over 900 incident male lung
cancer cases, analyses could be performed for separate strata of
smoking status and histology.

Materials and Methods

Subjects.

The Netherlands Cohort Study on diet and cancer is a prospective cohort
study that started in September 1986. The study design has been
reported in detail elsewhere (21)
. The cohort included
58,279 men of ages 55–69 years at the beginning of the study. The
study population originated from 204 municipalities with computerized
population registries.

Data Collection.

At baseline, the cohort members completed a mailed, self-administered
questionnaire on dietary habits, lifestyle, smoking, personal and
family history of cancer, and demographics. The dietary section of the
questionnaire was a 150-item semiquantitative food frequency
questionnaire. The questionnaire concentrated on the habitual
consumption of foods and beverages during the year preceding the start
of the study. The principal nutrients of interest in the design of the
questionnaire were energy, protein, fat, cholesterol, carbohydrates,
dietary fiber, alcohol, calcium, vitamin A, β-carotene, and vitamin
C. The questionnaire was validated against a 9-day diet record
(22)
.

Information on dietary supplement use was collected with an open-ended
question with space for four different supplements at most.
Participants were asked whether they used vitamin tablets, drops, or
other preparations during the 5 years before baseline.

Food Composition Tables.

Mean daily nutrient intakes were calculated using the computerized
Dutch food composition table (23)
. For calculation of the
intake of specific carotenoids, an additional food composition table
has been constructed recently (20)
, providing information
on the most important carotenoids: α- carotene, β-carotene, lutein+
zeaxanthin, lycopene, and β-cryptoxanthin. Briefly, foods that are
the main sources of carotenoids (e.g., vegetables) were
sampled and analyzed for α-carotene, β-carotene, lutein,
zeaxanthin, and lycopene. Some other foods, such as margarines, were
also analyzed to check data supplied by manufacturers. Values for all
other foods mostly were derived from recent literature based on the
same methods of analysis as we used. For mixed dishes, carotenoids were
estimated based on the recipe of preparation. Vegetables were sampled
in two to three periods across a year, and at each occasion a pooled
sample derived from seven different retailers was analyzed as described
previously (20)
. In the carotenoid food composition table,
lutein and zeaxanthin had to be taken together because most of the
literature sources used did not provide separate values for each of
these carotenoids. Vegetables, however, contain primarily lutein and
only minor quantities of zeaxanthin. Folate content was mainly derived
from the food composition tables of McCance and Widdowson
(24)
.

Record Linkage.

The method of record linkage to obtain information on cancer incidence
has been described previously (25)
. In short, follow-up
for incident cancer was established by computerized record linkage with
all regional cancer registries in the Netherlands and with PALGA, a
computerized national database of pathology reports. Completeness of
follow-up of cancer was at least 96% (26)
. After 6.3
years of follow-up, i.e., from September 1986 to December
1992, 1096 incident male lung cancer cases were identified.

Data Analysis.

The case-cohort approach was used for data analysis (27)
.
This means that cases are enumerated for the entire cohort, whereas the
person-years at risk for the entire cohort are estimated from a random
subcohort sample. After baseline exposure measurement, a subcohort of
1688 men was randomly sampled from the cohort and followed up
biennially for vital status information. No subcohort members were lost
to follow-up over the 6.3 years of follow-up.

After exclusion of lung cancer cases who reported prevalent cancer
other than skin cancer at baseline, lung cancer cases without
microscopically confirmed cancer, cases with lung cancer other than
carcinoma (sarcoma, lymphoma, and unspecified morphology), or with
in situ lung carcinoma, 1050 incident lung cancer cases were
identified. From the subcohort, prevalent cancer cases other than skin
cancer were excluded as well. Subjects with incomplete or inconsistent
dietary data were also excluded from analyses. These included subjects
who: (a) left blank 60 (of 150) items in the questionnaire
and reported to eat <35 items at least once a month or more; or
(b) subjects who left one or more item blocks (grouping of
items, e.g., beverages) blank. More details are given in a
separate report (22)
. Eventually, 939 male lung cancer
cases and 1525 male subcohort members were available for analyses.

Analyses were performed for the carotenoids α-carotene, β-carotene,
lutein + zeaxanthin, lycopene, and β-cryptoxanthin, vitamin C,
vitamin E, and folate. Mean intakes and SDs were calculated for cases
and subcohort members. For different strata of age, smoking status,
educational level, and family history of lung cancer, mean intakes were
calculated for the subcohort. Pearson correlation coefficients were
calculated for the food constituents of interest and energy intake. RRs
of lung cancer and their 95% CIs were computed using the GLIM
statistical package (28)
. Exponentially distributed
survival times were assumed in the follow-up period. Because standard
software was not available for case-cohort analysis, specific macros
were developed to account for the additional variance introduced by
sampling from the cohort instead of using the entire cohort
(29)
. Age, level of education, family history of lung
cancer, current cigarette smoking (yes/no), number of cigarettes per
day, and years of smoking cigarettes were considered as potential
confounders for which the RRs were adjusted. Because energy consumption
is frequently considered to confound RRs, an additional model was used
that included energy. Additionally, models were used adjusting for
either vitamin C or folate content, because these nutrients were found
to be highly correlated to (some of the) carotenoids. Tests for trends
in the RRs were based on two-sided likelihood ratio tests. Intakes of
food constituents were classified into quintiles based on the
distribution in the subcohort. For vitamin C, however, the validation
study had pointed out that quintiles 2 and 3 and quintiles 4 and 5
could not be distinguished; therefore, we reduced vitamin C intake to
three categories (22)
. RRs for quintiles/categories of
intake were also computed separately in never-smokers, former smokers,
and current smokers. Because the number of cases among never-smokers
was small, tertiles of intake were used instead of quintiles. Also,
subgroup analyses were performed for the most common histological types
of lung cancer: small cell carcinoma, squamous cell carcinoma, and
adenocarcinoma.

Results

For both lung cancer cases and subcohort members, mean daily
intakes of the major carotenoids, vitamin C, vitamin E, and folate are
presented in Table 1⇓
. In general, intakes were lower among cases than in the subcohort. Mean
daily intakes of the subcohort are presented for different categories
of age, smoking status, educational level, and family history of lung
cancer, which are potential confounders of the association between
intake data and lung cancer incidence (Table 2)⇓
. In the higher age categories, intake of carotenes and folate tended
to be lower and intakes of β-cryptoxanthin and vitamin C tended to be
higher (Table 2)⇓
. Current smokers had the lowest intakes for all
carotenoids and vitamins of interest, and subjects who had never smoked
had the highest intakes (except for vitamin E). For educational level,
intake data did not show clear trends, and subjects with a family
history of lung cancer tended to have lower intakes of most carotenoids
and vitamins of interest.

Because the carotenoids and vitamins partly originated from the same
food sources (vegetables and fruit), intakes could be highly
correlated. To illustrate this, Pearson correlation coefficients were
calculated for subcohort members (Table 3)⇓
. Correlations were high between α-carotene and β-carotene (0.93),
between lutein + zeaxanthin and β-carotene (0.68), between vitamin C
and β-cryptoxanthin (0.77), and between folate and eitherβ
-carotene (0.62), lutein + zeaxanthin (0.66), or vitamin C (0.66).
Also, correlation coefficients with energy intake were generally low
with the exception of vitamin E (0.53) and folate (0.58).

Pearson correlation coefficients of carotenoids, other antioxidant
vitamins, and folate in men from the subcohort of the Netherlands
Cohort Study 1986–1992

RRs for lung cancer were calculated with different models. The most
straightforward model, including age, smoking, socioeconomic status,
and family history of lung cancer, revealed significant inverse
associations with lung cancer incidence for lutein + zeaxanthin,β
-cryptoxanthin, vitamin C, and folate (RRs for highest
versus lowest category: 0.72, 0.71, 0.63, and 0.70,
respectively; 95% CI all excluding 1.00; and P-trends <0.05; Table 4⇓
). No significant effects were found for α-carotene, β-carotene,
lycopene, or vitamin E, although for the carotenes RRs were <1.00 for
all quintiles above reference. The use of supplements containing
vitamin C or vitamin E was not associated with the incidence of lung
cancer. Including energy intake in the model did not change RRs
importantly. Additional inclusion of vitamin C, however, reduced
the effects found previously, leaving only folate with a significant
negative trend. Adjustment for folate only left significant negative
associations for β-cryptoxanthin and vitamin C.

RRsa
(95% CI) for quintiles of dietary intake
of carotenoids, vitamins C and E, and folate for lung cancer, according
to different models: Netherlands Cohort Study 1986–1992

Cases and subcohort members were classified based on smoking status at
baseline: never-smokers (35 cases), former smokers (312 cases), and
current smokers (487 cases; Table 5⇓
). In current smokers, significant inverse associations were found forβ
-carotene, lutein + zeaxanthin, β-cryptoxanthin, vitamin C, and
folate, and the associations were stronger than those found in
unstratified analyses. In former smokers, significant inverse trends
were found only for lutein + zeaxanthin; all other RRs were closer to
1.00 than for current smokers. However, RR estimates for the carotenes,
folate, and vitamin C remained <1.00 for all quintiles/categories
above reference. With only 35 lung cancer cases among never-smokers,
estimates were unstable, despite the use of tertiles of intake instead
of quintiles. No significant trends were found for the carotenoids and
vitamins of interest. For β-cryptoxanthin, vitamin C, and vitamin E,
estimates were <1.00 for all categories of intake above reference, and
for the carotenes, they were >1.00. In analogy with data presented in
Table 4⇓
, additional adjustments were made for either vitamin C or
folate (results not shown). For current smokers, inverse relationships
with both carotenes and lutein/zeaxanthin disappeared, whereas negative
associations with β-cryptoxanthin, vitamin C, and folate remained
intact. Additional adjustment for vitamin C or folate left the negative
associations with lutein/zeaxanthin and folate in former smokers
unchanged. In never-smokers, additional adjustment for either vitamin C
or folate resulted in a stronger positive association with β-carotene
(results not shown).

RRsa
(95% CIs) for quintiles (Q) or tertiles
(T) of dietary intake of carotenoids, vitamins C and E, and folate,
among never-smokers, former smokers, and current smokers: Netherlands
Cohort Study 1986–1992

Lutein + zeaxanthin, β-cryptoxanthin, vitamin C, and folate showed
significant inverse associations with both risk of small cell
carcinomas (157 cases) and squamous cell carcinomas (377 cases; Table 6⇓
). Nonsignificant inverse associations were found for α-carotene andβ
-carotene with small cell carcinomas. Adenocarcinomas (143 cases)
seemed to be inversely associated only with folate intake and
(borderline) with vitamin C but not with the carotenoids. Similar to
analyses for all lung cancer cases taken together, additional
adjustment for vitamin C resulted in a loss of inverse associations
with lutein + zeaxanthin and β-cryptoxanthin, with the exception ofβ
-cryptoxanthin in squamous cell carcinomas, which stayed
significantly negative. The inverse association with folate disappeared
for small cell carcinomas but not squamous cell and adenocarcinomas.
Similar to analyses for the whole group, adjustment for folate weakened
the inverse associations for small and squamous cell carcinomas with
lutein + zeaxanthin but not with β-cryptoxanthin. The inverse
associations found with vitamin C were weaker for all histological
types of carcinomas after adjustment for folate. β-Carotene turned
out to have a significant positive relationship with adenocarcinomas
(results not shown).

Because folate and vitamin C were highly correlated, additional
analyses were performed with combinations of tertiles of folate
consumption with categories of vitamin C intake (Table 7)⇓
. For men who had high intake of both nutrients compared with men who
had a low intake, the RR was 0.56 (95% CI, 0.44–0.72). In the highest
tertile of folate intake, a higher vitamin C intake led to a lower RR,
and vice versa.

Discussion

In the Netherlands Cohort Study on Diet and Cancer, the
associations were evaluated between lung cancer incidence and intakes
of several carotenoids, vitamin C, vitamin E, and folate. After
adjustment for age, smoking history, education level, and family
history of lung cancer, statistically significant protective effects on
lung cancer incidence were found for the carotenoids lutein +
zeaxanthin and β-cryptoxanthin as well as for folate and vitamin C.
For other carotenoids (α-carotene, β-carotene, and lycopene) and
vitamin E, no such inverse associations were found. Use of supplements
containing vitamin C or vitamin E did not influence lung cancer
incidence. In our previous report on vegetable and fruit consumption
and lung cancer risk, consumption of vegetables and fruits appeared to
be inversely associated with lung cancer (5)
. The
protective effect on lung cancer was not limited to a specific group of
vegetables or fruits, but the effect was strongest for vegetables of
the Brassica genus and for citrus fruit. The present
analyses on carotenoids, other antioxidants, and folate suggest that
neither α-carotene nor β-carotene is mainly responsible for the
inverse associations found, but that contributions of lutein +
zeaxanthin, β-cryptoxanthin, folate, and vitamin C are more
important. The RRs for these food constituents, comparing the highest
category of intake with the lowest one, are of the same magnitude as
the ones found for vegetable and fruit intake. The relatively low RR
for vitamin C (0.63; 95% CI, 0.49–0.83) is in line with the lower RRs
for oranges and fresh orange juice, as published before.

There may be some limitations to this study which could have influenced
the results. With 6.3 years of follow-up and 939 male lung cancer
cases, the power of the study is such that the probability for overall
observed results to be caused by chance is small. The prospective
nature of a cohort study together with completeness of follow-up, as
has been achieved in this study, reduced the potential for selection
bias to a minimum. Information bias, i.e., a change in
(reported) dietary habits of lung cancer cases because of the disease,
is also largely avoided in a prospective study because dietary habits
were reported before the disease was diagnosed. A change in dietary
habits of subjects with latent lung cancer at the time of completing
the baseline questionnaire remains, however, a possibility, although
this is much less likely than in subjects with gastrointestinal cancer,
for example. Repeating analyses excluding cases diagnosed during the
first year of follow-up hardly affected RRs (results not presented).

Intake data were based on the baseline questionnaire in 1986, departing
from the assumption that dietary and other habits would not change
considerably during the 6.3 years of follow-up. Because participants
were 55–70 years of age at entry, important changes were not expected.
In a random sample of the cohort, the reproducibility of the
questionnaire was tested during the first 5 years of follow-up, which
led to the conclusion that the potential of a single food frequency
questionnaire measurement to rank subjects according to nutrient intake
dropped only slightly over time (30)
. Measurement error
attributable to use of the carotenoid database will be relatively
small, because analyses were performed in those vegetables known to be
mainly responsible for carotenoid intake in the Dutch diet.

A potentially more realistic threat to the interpretation of the
observed inverse associations is residual confounding by risk factors
for lung cancer that happen to be associated with the intake of
carotenoids, vitamin C, vitamin E, or folate. The most important risk
factor in this respect is cigarette smoking. Unhealthy habits, such as
smoking and a diet low in vegetables and fruit, tend to cluster in the
same subjects in most populations (31)
; therefore,
insufficient control of one factor will confound the association
between the other factor and lung cancer. In this way, it might happen
that an observed inverse association between intake of food
constituents and lung cancer was attributable to residual confounding
by smoking. Evidence of an association between smoking and intake of
several nutrients was also found in the Netherlands Cohort Study of
Diet and Cancer, although it was not very strong and not very
consistent. For example, never-smokers had higher intakes of all
carotenoids, vitamin C, and folate than former smokers, and current
smokers had the lowest intakes for all food constituents of interest.
To minimize residual confounding attributable to smoking, we made an
attempt to model cigarette smoking habits such that they best explained
lung cancer. This resulted in a model including number of years smoked,
habitual number of cigarettes smoked per day, both as continuous
variables, and an indicator variable for current smoking. When we added
the smoking variables to an age-adjusted model, the RR estimates for
food constituents and lung cancer changed only slightly (data not
shown). We believe, therefore, that the inverse association observed
between the food constituents of interest and lung cancer was not
entirely attributable to residual confounding by smoking, although we
cannot exclude some influence.

In this study, adjustment for either vitamin C or folate intake reduced
the effects of carotenoids importantly. This led to the conclusion that
vitamin C and folate might be more important in terms of protection
against lung cancer incidence than the carotenoids. Using combinations
of men high in intake of both nutrients versus men with a
low intake, we could conclude that the effects of vitamin C and folate
are independent of each other.

In the past decade, five cohort studies have reported RRs forβ
-carotene, three showing nonsignificant inverse associations with
lung cancer (14, 15, 19)
, one a nonsignificant inverse
association for women but no association for men (13)
, and
one a significant inverse trend only for subjects with stable high
intakes over a 10-year period (16)
. One cohort study
recently published data on individual dietary carotenoids, showing a
statistically significant inverse association for α-carotene only.
For total carotenoids, significant negative associations have been
found for men only in the New York State Cohort Study (18)
and for nonsmoking men in the Finnish Mobile Clinic Health Survey
(12)
. Nonsignificant negative associations have been found
in policy holders with the Lutheran Brotherhood Insurance Society
(14)
and in the First National Health and Nutrition
Examination Survey Epidemiological Follow-up Study (17)
.
Two cohort studies have recently reported RRs for vitamin C of the same
magnitude as those we found (RR 0.63 for the highest versus
the lowest category; Refs. 17
and 18
). Others
found inverse nonsignificant (14, 15)
or nonconsistent
(16)
RRs for vitamin C, one only found a nonsignificant
inverse association for women but no association for men
(13)
, and one study found a strong inverse relationship
for nonsmokers only (12)
. A statistically significant
positive association for vitamin C was found recently in the Nurses’
Health Study (19)
, but supplements were included,
resulting in much higher intakes in the highest quintiles than in our
study. No data were presented excluding supplement use. As in our
study, no effect was found for vitamin E in all five studies that
included this nutrient (12, 16, 17, 18, 19)
. In our study, among
the nutrients included, folate appeared to be the strongest and most
consistent protective factor against lung cancer incidence. A highly
significant inverse association for folate was found in the New York
State cohort as well (18)
, but the Nurses’ Health Study
found no association (19)
. Again, however, in the latter
study, supplements were included. Intake levels of the fourth quintile
in the NHS were comparable with the fifth quintile level in our study
(about 400 μg/day), and the RR of this fourth quintile of the NHS
(compared with the first) was 0.7 (95% CI, 0.5–0.9). A negative
association with folate may be an important finding, because folate
possibly has a role in carcinogenesis, given its involvement in DNA
synthesis, methylation, and repair (32)
. However, thus
far, it has rarely has been associated with the incidence of lung
cancer.

As summarized by Van Poppel and Goldbohm (33)
, cohort
studies using plasma concentrations of β-carotene have found
remarkably consistent negative associations with lung cancer. This
could be in line with our results, because plasma β-carotene can be
considered to be a marker for folate, vitamin C, or other carotenoids.
In a prospective study published recently, levels of several
carotenoids and other antioxidants in blood were linked to lung cancer
risk (34)
. Protective effects on lung cancer were found
for β-cryptoxanthin, β-carotene, and lutein + zeaxanthin.
Nonsignificant protective effects were found for α-carotene and
ascorbic acid. No effect was found for lycopene, α-tocopherol, and
selenium.

Negative associations were strongest among current smokers at baseline
and weaker for former smokers. Only 35 cases were never-smokers, and no
significant associations with the carotenoids and vitamins mentioned
could be found. These results are contradictory to results in Finnish
men (117 lung cancer cases), where inverse gradients were found for
summed carotenoids (α- carotene, β-carotene, γ-carotene,
lycopene, and lutein + zeaxanthin), vitamin E (nonsignificant), vitamin
C, and lung cancer incidence only in nonsmokers. However, in that
study, nonsmokers included both former smokers (18 cases) and
never-smokers (6 cases). Among smokers (93 cases), no associations were
found (12)
. More in line with our results are those from
the National Health and Nutrition Examination Survey Epidemiological
Follow-up Study, showing a protective effect of carotenoids and vitamin
C only among current smokers, not in nonsmokers (never-smokers + former
smokers). In that study, significant inverse associations were found
only in the lowest tertiles of pack-years of smoking for carotenoids,
vitamin C, and vitamin E (17)
. In the Iowa Women’s Health
Survey, separate analyses were made on β-carotene for current smokers
(81 cases), ex-smokers (38 cases), and never-smokers (19 cases).
Because of the small number of cases, conclusions were hard to draw,
but the association seemed to be stronger in former than in current
smokers (15)
. In the New York State Cohort, only analyses
have been presented for light and heavy smokers separately. Risk
estimates for vitamin C and carotenoids were of similar magnitude for
light and heavy smokers. The effect of folate appears to be the
strongest in heavy smokers (18)
. As shown in this study,
estimates change after correction for either vitamin C or folate.
Comparisons with other studies have to be made without further
adjustment, because other groups have not presented adjusted results.
In summary, results of different studies are hard to compare because
analyses were based on different definitions of smoking strata. In
general, results on never-smokers are not relevant thus far in cohort
studies, because insufficient cases had been diagnosed to do proper
analyses. However, as summarized by Ziegler et al.(4)
, case-control studies that focused on never smokers
found effects of carotenoids, vegetables, and fruit similar in
magnitude to those generally observed among active smokers.

In the present study, negative associations with carotenoids seemed to
be limited to small cell carcinomas and squamous cell carcinomas. Both
folate and vitamin C intake appeared to be inversely related to small
cell, squamous cell, and adenocarcinomas. Two other cohort studies have
presented results stratified by histological type of lung cancer. The
New York State Cohort Study analyses were performed separately for
squamous cell carcinomas and adenocarcinomas, indicating significant
inverse associations of vitamin C and folate with squamous cell
carcinomas only (18)
. In the Iowa Women’s Health Study,
analyses on β-carotene were performed separately for adenocarcinomas,
small cell carcinomas, squamous cell carcinomas, and large cell
carcinomas, revealing only a (nonsignificant) negative association for
small cell carcinomas (37 cases) and large cell carcinomas (12 cases).
Because of the small numbers of cases, these latter results might be
less accurate (15)
. Results of the New York State Cohort
Study are in line with our results, although it should be stressed that
in our study, additional adjustment for either folate or vitamin C
importantly influenced risk estimates. A lack of effect of carotenoids,
vitamin C, and folate on adenocarcinomas might be attributable to the
fact that these are less strongly related to smoking (18)
.

By comparing our findings with available results of cohort studies, we
conclude that carotenoids, especially β-carotene, and vitamin E have
only marginal inverse associations with lung cancer incidence. This is
in line with results of the three major published intervention trials
that have not found a reduction in lung cancer incidence in subjects
given vitamin E (35)
, vitamin A (36)
, or β-
carotene (35, 36, 37)
. However, our study has indicated that
lutein + zeaxanthin and β-cryptoxanthin, as well as vitamin C and
folate, might be more promising food components in this matter,
although associations with both carotenoids did not persist after
further adjustment with vitamin C or folate.

Results from this prospective cohort study indicate that α-carotene
and β-carotene appear not to be the food constituents responsible for
a lower lung cancer incidence with higher vegetable and/or fruit
consumption. The role of other important carotenoids (if any) may be
limited to lutein + zeaxanthin and β-cryptoxanthin, and more studies
are required to allow definite conclusions to be drawn. Because both
vitamin C and folate appear to have a stronger protective impact on
lung cancer risk, these should be included in future analyses. It
remains possible, however, that inverse associations with lung cancer
risk are a generalized fruit and vegetable effect and cannot be
attributed to one or more specific nutrients.

Footnotes

The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked advertisement in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

↵1 This study was financially supported by the
Dutch Cancer Society and the Dutch Product Board for Horticulture.

The α-Tocopherol, β-Carotene Cancer Prevention Study Group The effect of vitamin E and β-carotene on the incidence of lung cancer and other cancers in male smokers. N. Engl. J. Med., 330:1029-1035, 1994.